The method and coating compositions of the present invention are primarily directed to the provision of coatings for cold-rolled, non-oriented, fully-processed electrical steels and cold-rolled, non-oriented, semi-processed electrical steels. The term "cold-rolled, non-oriented, fully-processed electrical steels", as used herein and in the claims, is intended to refer to those electrical steels subjected at the mill to a quality anneal involving grain growth which may include decarburization to develop optimum magnetic properties. The term "cold-rolled, non-oriented, semi-processed electrical steels", as used herein and in the claims, is intended to refer to those electrical steels not given a quality anneal at the mill to fully develop magnetic properties. The customer may complete the processing by a quality anneal. This quality anneal involves grain growth and possibly decarburization (depending upon the amount of decarburization accomplished at the mill), essential to the development of optimum magnetic properties. Both semi-processed and fully-processed electrical steels, generally used for transformer, motor or generator laminations, are silicon steels. Nevertheless, the term "cold-rolled, non-oriented, semi-processed electrical steels" is intended to be inclusive of semi-processed carbon steels for motor laminations and the like.
The American Iron and Steel Institute publication "STEEL PRODUCTS MANUAL--ELECTRICAL STEELS", (January 1983, pp. 15-17), defines various insulative coatings for electrical steels and categorizes them as C-0, C-2, C-3, C-4 and C-5. A C-3 coating is an organic coating (usually a varnish or the like) designed for punchability. While resistant to normal operating temperatures up to 300.degree. F. (185.degree. C.), a C-3 coating will not provide adequate surface insulation after a stress relief anneal and may begin to deteriorate at temperatures above about 750.degree. F. (400.degree. C.). A C-5 coating is an inorganic insulative coating which will provide adequate surface insulation after a stress relief anneal. However, because it is inorganic, it does not have the punching qualities of a C-3 coating.
Fully-processed and semi-processed steels of the type defined above can be provided at the mill with an insulative coating. If the fully-processed laminations are not going to be annealed and, in service, will not be exposed to temperatures above 300.degree. F. (185.degree. C.), an organic or AISI Core Plate 3 coating can be utilized. In this application, the punching die wear and weldability characteristics of the coating can constitute important considerations. When the fully-processed laminations are stress relief annealed, the ability of the coating to provide adequate surface insulation after the anneal is important. A typical stress relief anneal is practiced at about 1400.degree. F. (760.degree. C.) to 1550.degree. F. (843.degree. C.) in an atmosphere which is non-oxidizing or neutral with respect to iron, such as 95% nitrogen and 5% hydrogen.
With respect to semi-processed steels, punching or the like is usually followed by a quality anneal (which also serves as a stress relief anneal) conducted in a decarburizing or neutral atmosphere containing components such as water vapor, hydrogen, nitrogen, hydrogen-nitrogen, or an atmosphere formed by partial combustion of gas. The quality anneal is usually conducted at a temperature within the range of from about 1400.degree. F. (760.degree. C.) to 1650.degree. F. (900.degree. C.). Under these circumstances, a Core Plate 3 coating cannot be used and a primary characteristic of the coating to be considered is its anti-stick characteristics. Additional characterstics often looked for is the coating's ability to prevent nitrogen pick-up and surface oxidation during a quality anneal, together with some rust protection for the laminations both prior to and after the quality anneal.
There are other factors which come into play with respect to such coatings. For example, motor manufacturers usually coat the motor windings with an epoxy or the like. In order to rebuild a motor and replace its windings, it is necessary to burn-off this epoxy coating. Such a burn-off anneal is usually conducted in air at a temperature above about 750.degree. F. (400.degree. C.). It is desirable that the coatings on the motor laminations provide adequate surface insulation after such a burn-off anneal.
Prior art workers have searched for a coating which could replace C-3 and C-5 coatings. In other words, a single coating which would have improved punchability compared to a C-5 coating, while being capable of providing adequate surface insulation after heat treatments at elevated temperatures.
Prior art workers have devised numerous inorganic-organic coatings for metal for various purposes. In fact, U.S. Pat. Nos. 3,908,066 and 3,839,256 teach coatings for providing electrical insulation when applied to electrical steel sheets for magnetic cores. The coating compositions taught therein consist of an organic portion comprising aqueous dispersions of copolymers, terpolymers or ionomers of ethylene and ethylunically-unsaturated carboxylic acids or esters blended with organic quaternary ammonium-silicate solutions. Nevertheless, to date there has not been developed an inorganic-organic coating capable of providing surface insulation after a stress relief anneal equivalent to a C-5 coating after a stress relief anneal.
U.S. Pat. Nos. 3,996,073 and 3,948,786 disclose insulative coatings which may be used in addition to or in place of a mill glass on electrical steels, such as cube-on-edge oriented silicon steel. The teachings of these patents are incorporated herein by reference. A coating solution, in accordance with these patents, contains aluminum, magnesium and phosphate in the following relative relationship on a water-free basis:
From 3% to 11% by weight Al.sup.+++ calculated as Al.sub.2 O.sub.3, from 3% to 15% by weight Mg.sup.++ calculated as MgO, and from 78% to 87% by weight H.sub.2 PO.sub.4.sup.- calculated as H.sub.3 PO.sub.4, with the total weight percentage of Al.sup.+++ (as Al.sub.2 O.sub.3), Mg.sup.++ (as MgO) and H.sub.2 PO.sub.4.sup.- (as H.sub.3 PO.sub.4) being 100% on a water-free basis. The concentration of Al.sup.+++, Mg.sup.++ and H.sub.2 PO.sub.4.sup.- comprising 100 parts by weight calculated as aluminum oxide, magnesium oxide and phosphoric acid, respectively, on a water-free basis. In the embodiment of interest, colloidal silica is present within the range of 33 to 150 parts by weight on a water-free basis. At least 60% by weight of the coating solution must be water. From 10 to 25 parts by weight chromic anhydride for every 100 parts by weight H.sub.2 PO.sub.4.sup.-, calculated as H.sub.3 PO.sub.4 are added in order to stabilize the colloidal silica and to provide satisfactory adherence, and lack of hygroscopicity and "tack" after curing at about 250.degree. F. (120.degree. C.) to about 350.degree. F. (175.degree. C.).
A commonly owned co-pending application, Ser. No. 06/612450, filed 5/21/84, in the name of Michael H. Haselkorn, and entitled "INSULATIVE COATING COMPOSITION FOR ELECTRICAL STEELS", teaches coating compositions similar to those set forth in U.S. Pat. Nos. 3,840,378 and 3,948,786, but in which both colloidal silica and chromic anhydride have been eliminated. The teachings of this co-pending application are incorporated herein by reference. Briefly, the coating solution contains aluminum, magnesium and phosphate in the same relative relationship as taught in the above noted U.S. Pat. Nos. 3,996,073 and 3,948,786 and enumerated above. In addition, the coating solutions contain aluminum silicate.
The present invention is based upon the discovery of a method and coating compositions for forming coatings having improved punchability compared to a C-5 coating, while being capable of providing adequate surface insulation after heat treatments at elevated temperatures. Such coatings are intended for use on cold-rolled, non-oriented fully-processed electrical steel, cold-rolled, non-oriented semi-processed electrical steel, and cold-rolled, non-oriented motor lamination steel. The coating compositions contain an inorganic portion and an organic portion. It has been found that the coating compositions taught in the above-mentioned U.S. Pat. Nos. 3,996,073 and 3,948,786 or the coating compositions taught in the above noted co-pending application can serve with excellent results as the inorganic portion of the coating compositions of the present invention. In addition to either of these inorganic portions, an organic portion is added, consisting essentially of a water soluble or dispersible resin containing by weight at least 40% water.
The coatings of the present invention are also particularly applicable to punching quality oriented electrical steel. Such steel is normally punched and stress relief annealed by the customer. During the manufacturing process, a mill glass is formed on the steel during the high temperature anneal in which the grain orientation is developed. The mill glass is removed by pickling since it would be harmful to punching dies. The steel manufacturer normally coats the pickled steel with a C-5 coating to provide surface insulation both before and after stress relief anneal, although such a coating is also somewhat harmful to punching dies. Application of the coatings of the present invention to such punching quality oriented electrical steel provides adequate insulation after stress relief annealing (similar to a C-5 coating) while providing improved punchability (similar to that of a C-3 coating).